Re: [PATCH 0/3] x86: clear vmcss on all cpus when doing kdump ifnecessary

From: Zhang Yanfei
Date: Wed Oct 17 2012 - 21:13:21 EST

ä 2012å10æ17æ 18:16, Avi Kivity åé:
> On 10/17/2012 04:28 AM, Zhang Yanfei wrote:
>> ä 2012å10æ15æ 23:43, Avi Kivity åé:
>>> On 10/12/2012 08:40 AM, Zhang Yanfei wrote:
>>>> Currently, kdump just makes all the logical processors leave VMX operation by
>>>> executing VMXOFF instruction, so any VMCSs active on the logical processors may
>>>> be corrupted. But, sometimes, we need the VMCSs to debug guest images contained
>>>> in the host vmcore. To prevent the corruption, we should VMCLEAR the VMCSs before
>>>> executing the VMXOFF instruction.
>>> How have you verified that VMXOFF doesn't flush cached VMCSs already?
>> I tried some tests, for example, I made copies for every vmcs, and in the kdump
>> path, I backed up all the loaded vmcs into the copies before vmxoff.
>> After generating the vmcore, I retrieve the vmcss and their copies, and compare them,
>> no differences.
>> Another test is using VMCLEAR to clear all the loaded vmcs before VMXOFF,
>> and compare the vmcss and their copies, there are indeed differences between the
>> vmcs and its copy.
>> I know the tests may be not so convincing, for example, I used memcpy to back up
>> the vmcss and it is an ordinary memory operation. But to ensure the non-corruption
>> of the vmcss in the vmcore, I think we should VMCLEAR the vmcss before VMXOFF just
>> as the Intel spec says.
> Sorry, I was unclear -- I was referring to the spec, I wasn't sure
> whether VMXOFF is defined to flush VMCSes or whether it just invalidates
> on-chip caches so that it won't flush them out in the future, corrupting
> memory. We don't want to depend on actual behaviour as it may change
> with future version.
> Copying some Intel folk, maybe they can clarify it.

Yes, the Intel spec says "may be" about the VMCS-corruption thing. From
chapter 24.10.1 in Intel 64 and IA-32 Architectures Software Developerâs
Manual Volume 3C:System Programming Guide, Part 3, there is the description:

"If a logical processor leaves VMX operation, any VMCSs active on that logical
processor may be corrupted (see below). To prevent such corruption of a VMCS that
may be used either after a return to VMX operation or on another logical processor,
software should VMCLEAR that VMCS before executing the VMXOFF instruction or
removing power from the processor (e.g., as part of a transition to the S3 and S4
power states)."

Our purpose is to make sure the VMCSs in the vmcore are updated and non-corrupted. So
according to the description above, no matter whether VMXOFF is defined to flush
VMCSs or whether it just invalidates on-chip caches, we'd better VMCLEAR the
VMCSs before executing the VMXOFF.

Zhang Yanfei
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